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Shingle Sidewall

jnarchitects | Posted in Green Building Techniques on


Working on a detail with a builder to add 1″ of polyiso to the exterior of an existing 2×4 wall in Zone 5.

Planning to install cedar singles as the sidewall material. Most often, it seems the drainage layer (water control layer) is behind the foam (b/w foam and sheathing). However, we would like to use a drain wrap as the air control layer with the added benefit of providing a small (very small) level of drain space behind the shingles.

Any issues with placing the drain wrap b/w the shingles and foam as opposed to installing the housewrap between foam and sheathing?

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  1. bdrfab | | #1

    No problem with having the WRB on the exterior of your foam. However, 1" of polyiso in Climate zone 5 is insufficient exterior insulation. If you search the site there are numerous articles on how much insulation is needed, as well as integrating a WRB into your system.

  2. GBA Editor
    Martin Holladay | | #2

    I disagree with Aaron on the question of whether 1 inch of polyiso (R-6) is adequate for a 2x4 wall in Climate Zone 5. In fact, all you need is a minimum of R-5 for your exterior foam, so your exterior rigid foam is thick enough. For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

    There is a more important problem to your plan, however. I don't recommend installing cedar shingles on rigid foam. Since the shingles won't be able to dry quickly on the back side, they will tend to curl and split if you nail them over foam.

    The usual methods on homes with exterior rigid foam are either (a) install a layer of OSB or plywood over the foam (some builders use nailbase), or (b) install the cedar shingles on horizontal 1x3 strapping.

    If you choose to install plywood or OSB over the foam, the best installation method will include a three-dimensional drainage mat like Cedar Breather.

  3. bdrfab | | #3

    Apologies, I had the 7.5 number in my head. Sorry to get off tangent but what is the reasoning behind lower R numbers on 2x4 walls?

  4. GBA Editor
    Martin Holladay | | #4

    If you build a wood-framed wall with exterior rigid foam, the wall sheathing is sandwiched between two layers of insulation. The temperature of the sheathing is somewhere on the continuum of temperatures between the outdoor temperature and the indoor temperature.

    The thicker the wall framing (and the thicker the insulation installed between the wall studs), the closer the wall sheathing is to the outdoor temperature. In other words, if you have 2x6 walls, the sheathing will be colder in winter than if you have 2x4 walls -- unless you make the rigid foam thicker.

  5. jnarchitects | | #5


    Would you have the same concerns with cedar shingles directly over housewrap or felt?

    Just wondering if you see a specific difference when foam is included...


  6. bdrfab | | #6

    At a bare minimum, best practice is to install cedar shingles over a "drain wrap" house wrap, often called crinkle wrap around here. Better yet is Cedar Breather as Martin described, however it is expensive and in my experience kind of a pain in the rear to work with.

  7. GBA Editor
    Martin Holladay | | #7

    Aaron's answer is correct. But if you install cedar shingles over a layer of asphalt felt, and if the asphalt felt is installed over a layer of board sheathing or plywood sheathing, then you have some moisture-buffering capacity in the asphalt felt and the sheathing. There materials are hygroscopic, and can therefore take on moisture when the shingles are wet. This is useful.

    On the other hand, rigid foam can't take on any moisture at all. That's why rigid foam is the worst possible substrate for cedar shingles or any wood siding.

  8. jnarchitects | | #8

    Hi Martin and Aaron,

    Thanks for the response. We typically use cedar breather on sidewall. It is so expensive, I was going to price a drain wrap as an alternative.

    In our coastal environment, we have been getting 25-30 yrs. out of cedar shingles over felt. I have wondered how many years you gain with cedar breather. Is it 5 or 10? Just wonder if the upcharge for the product is worth the long term benefits? I suppose the drainage space is not just beneficial for the siding, but for the substrate as well. But still wonder if there is truly a equal benefit to the cost difference.

    Thanks as always.

  9. Joe_The_Builder | | #9

    Martin and Jill,
    I am in the same climate area, but technically it’s zone 5. I am planning to use asphalt felt over plywood and shiplap wall sheathing to replace sections of cedar sidewall shingles that need to be replaced. This method doesn’t address the lack of air barrier. We will have Sheetrock inside, does this qualify as the air barrier? Or is there something else I should be doing to make an air barrier. Since this is a remodel, as long as it won’t cause issues, I’m fine with simply replacing the asphalt felt and only having a vapor barrier.

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